Integrated optic phase modulators are becoming widely used for frequency shifting in many optical devices. One application for such a modulator is to energize it with a sawtooth waveform in order to effect a desired phase shift in a light signal. This may be done for closed-loop or even for open-loop control of a system. Particular applications for such a "serrodyne" driven modulator may include fiber optic gyros, communications circuits, coherent light based sensors, etc. An example of such an application (fiber optic gyro) developed by applicants is presented below, but others are available and are within the scope of the invention.
In fiber optic gyros a coherent light beam is split in two component beams which are launched into each end of an optical fiber coil to propagate in opposed directions until recombined, after traversing the coil, to form an interference phenomenon, e.g., a fringe pattern at a detector. When subjected to a rotation having a component perpendicular to the plane of the coil, a nonreciprocal (acting on one beamd but not the other) phase shift, known as the Sagnac effect, is induced between the two counterpropagating beams thus causing a change in the interference phenomenon, e.g., a shift in a fringe pattern. The magnitude and direction of the fringe shift is proportional, respectively, to the rate and sense of the rotation applied to the coil about the axis.
The Sagnac effect can be understood in layman's terms by considering two examples, first a simple case dealing with the behavior of light in a straight path and then in a rotational path.
First, consider two pickup trucks, one following the other along a highway, both at the same speed, with a man in the trailing vehicle's bed throwing a first baseball to a man standing in the leading vehicle's bed at the exact moment the trailing truck passes another man standing on the side of the road who also, at that moment, throws a second baseball to the man in the leading truck. The first baseball will travel through the air from the trailing truck to the leading truck at the common speed of the trucks plus the velocity with which it was launched. Assuming equal launch velocities and neglecting frictional effects, the second baseball will clearly lag the first due to its only having the velocity with which it was launched. Now consider the same two trucks traveling at the same constant speed along the same route again but at night. The man in the trailing vehicle has a flashlight and the man in the leading truck a light detector. Similarly, the man on the side of the road has a flashlight. As the trailing truck passes the man on the road, both flashlights are turned on. Because the speed of light in an isotropic medium is independent of the reference frame, the man with the detector will receive the two beams at the detector at the same time.
Now, for the second example. Starting with a non-light analogy to a fiber optic gyro (by which the Sagnac effect can be better distinguished for the layman), imagine a very small man standing on the edge of a rotating disc of a record player having a large metal tube on the top edge around its periphery except in the space occupied by the man. The man is holding two pistols and is standing in between the two openings of the tube. If he shoots the guns at exactly the same moment into each of the openings, quickly gets out of the way and inserts a target in the space between the openings, he can observe the two bullets exiting the tube at the same time and hitting the target simultaneously. On the other hand, if the tube is an optical fiber and he uses two flashlights and a detector instead of pistols and a target, the light beam propagating in the direction of rotation of the disc will take a longer amount of time to get to the detector than the beam in the other direction because it has further to go. Due to light by nature being independent of any reference frame, the beam in the direction of rotation has to travel a longer distance than the beam in the opposite direction. The distance for each beam is different from the length of the optical fiber by the amount the disc moved during the transit time of the particular beam. These differences may be manifested by allowing the beams to recombine so as to interfere with varying degrees of constructive and destructive interference, creating interference fringes which change their magnitude and sense according, respectively, to the speed and direction of the disc.
The phase difference between the two beams can be "nulled" by imposing a further nonreciprocal phase shift on the beams in the coil using the detector's output in conjunction with a phase modulator. Conceptually, "nulling the coil" is akin to use of a doctor's scale in which the patient's weight creates an imbalance which may be nulled by sliding various weights about until a null position is achieved, at which point the scale may be read. In fiber optic gyro applications, unlike the doctor's scale, the "patient's weight," i.e., the angular rotation rate about the gyro's axis may be changing continually. The Sagnac effect can be compensated or nulled automatically by means of a control circuit which detects the magnitude and sense of the fringe shift and provides a drive signal which drives the modulator to provide the required phase compensation to null out the shift. One can then measure the degree of phase compensation required as being directly proportional to rate.
Various modulators are known in the art including integrated-optic phase modulators. Such a modulator might typically comprise a lithium niobate crystal indiffused with titanium or might in general comprise an optical material capable of having its index of refraction changed by means of a voltage induced electric field applied thereto. The field may be applied with differing polarity in order to raise or lower the refractive index above or below the quiescent (nonenergized) index (or conceivably even some selected index above or below the quiscent index). A single modulator is typically located near or at one end of the coil in order to be in a position to shift the phase of one component beam as it enters the coil and the other as it leaves the coil. If the modulator were located midway in the coil the effect would be simultaneous to both exiting beams, and would cause no effect. However, the desired nonreciprocal effect is caused not only by the nonsymmetrical positioning of the modulator, which alone would not cause the disired effect, but by the fact that the voltage applied to the modulator is varied, typically in ramp fashion, so that a pair of light packets entering the separate end of the coil at the same time will propagate through the modulator at different time, when it has different indices of refraction. They will thus be subject to different degrees of delay.
A closed-loop drive circuit for a phase modulator for a fiber optic gyro may be viewed conceptually as causing the phase modulator to effectively either "lengthen" or "shorten" to coil (depending on the direction of rotation of the gyro) for both beams, but to different degrees in order to cause the two beams to always "see" or effectively traverse the same length of coil and exit the loop at the same time. Of couse, as discussed in the preceding paragraph, in reality there is no change in the length of the coil. Rather, the effect is accomplished by continually changing the index of refraction of a small part of the length of the loop (occupied by the modulator), thus imposing differing degrees of delay on the two beams. The magnitude the sense of the modulating voltage necessary to thus null the coil in closed-loop fashion is directly proportional to the rotation rate of the gyro. If a ramp voltage is used to energize the modulator its slope should be set up so as to change in proportion to changes in the rate of rotation of the gyro. A positive slope will indicate rotation in a direction opposite from that indicated by a negative slope.
If the index of refraction of the crystal modulator could be increased or decreased without limit and if a voltage source of infinite magnitude in both polarities were available it would be possible to increase or decrease modulation to null the coil for as long as necessary, without limit. (This would be akin, in the doctor's scale example, of the "patient'weight" increasing beyond the capabilities of the scale to measure). But of course this is not the case, and a constant magnitude sawtooth-type waveform is usually applied instead, the frequency being indicative of rate of rotation, with the slope changing polarity to indicate a change in direction of rotation.
(For an early example of a sawtooth waveform used to achieve frequency translation, see U.S. Pat. No. 2,927,280. That patent discloses the efficiency advantage of sawtooth modulation (near 100%) of a klystron versus the then prior art method of sinusoidal modulation (around 34% because of the need to use filters to reject unwanted frequencies). Although of general interest, no particular sawtooth drive circuit is disclosed in detail.
The use of a sawtooth-type waveform for modulation purposes presents certain difficulties which are in general related to the waveform's periodic discontinuity. The most important of these are the duration of the discontinuity itself and unwanted harmonics attributable to a nonzero duration of the discontinuity. Of course, it is desirable to minimize both. The deleterious influence of a finite flyback period (in a fiber optic gyro) on the proportional relation between the shift required to null the coil and the rotation rate is investigated in a letter authored by A. Ebberg and G. Schiffner appearing in the Jun. 1985 issue of Optics Letters, Vol. 10, No. 6, entitled, "Closed-loop fiber-optic gyroscope with a sawtooth phase-modulated feedback." In that article, it is implied that a nearly ideal sawtooth-type waveform flyback period would be about two percent of the whole period (See FIG. 6 of the letter, the accompanying caption and text on pp. 301-302).
A sawtooth generator for use in such closed-loop control circuits having a so-called, near-ideal 2% flyback period might typically comprise a capacitor, a discharge switch and a phase modulator connected in parallel across an opamp to form a dischargeable integrator responsive to a feedback signal indicative of the magnitude of the modulating voltage. The voltage across the capacitor of the integrator (appearing also across the modulator) is discharged whenever the output of the integrator is sensed by a comparator as having exceeded a selected level corresponding to the desired constant amplitude of the sawtooth. With this arrangement the frequency of the sawtooth will be directly proportional to the rate of rotation of the gyro and a digital frequency counter can be hooked up to readout the frequency which can in turn be converted to angular rate or to pure angle. However, because the phase modulator is driven by the low impedance output of the op amp, closing the switch will result in a surge of current from the output of the op amp. This surge will interfere with the discharging of the capacitor, causing unwanted harmonics and increasing the flyback period. The flyback period and the unwanted harmonics can only be minimized by using super fast op amps, which are very expensive.
Another approach which might be used in the prior art would be a closed loop version of what is shown in U.S. Pat. No. 3,952,306, issued to Benton, for generating a constant peak amplitude, variable frequency sawtooth wave. That circuit uses the output of a sweep genertor (e.g., a sawtooth generator) to drive both a current source, which charges a capacitor to provide the variable frequency sawtooth ramp, and a voltage controlled multivibrator to control the frequency at which the capacitor is discharged through a discharge circuit. The sawtooth output voltage on the capacitor is provided to a buffer amplifier in order to present a low impedance to the load.
(Also discussed in that patent is what is described as the then prior art serrodyne method of using a current source to linearly charge a capacitor and a unijunction transistor to set a threshold level and discharge the capacitor. The current source was varied in order to change the sawtooth frequency. This proved satisfactory at frequencies up to 10 kHz, however, due to the delay time of the unijunction, at frequencies above 10 kHz, a variation in amplitude occurred as the frequency was swept {see column 1, lines 20-28}).
It is well understood in the art of frequency shifting with serrodyne modulation that when the ramp is reset, a disturbance exists in the detector for the duration of the loop light propagation delay if the peak value of the sawtooth waveform of absolute phase shift does not equal an integral number of wavelengths (2 radians of phase shift) of the coherent light. In other words, the maximum amplitude of the phase modulation imposed at the end of each sawtooth period must exactly equal one or more whole wavelengths of the light propagating in the fiber coil. If the instantaneous phase modulation at the time of reset is greater or less than one coherent ligth wavelength then the counterpropagating light waves will not experience reset at exactly the same relative point in the coherent period. This would result in the modulation beginning earlier or later each coherent cycle with respect to the other, thus introducing a phase error. One suggestion to deal with this error in the phase nulled output is to gate the receiver from the error and use the signal from the ramp time to control the period of the serrodyne ramp waveform. The error signal from the reset period can also be used to control the V.sub.2pi threshold, in a second control loop. See "Double Closed Loop Hybrid Fiber Gyroscope Using Digital Phase Ramp," by H. C. Lefevre, P. H. Graindorge, H. J. Ariditty, S. Vatoux and M. Papuchon. According to the article, the ratio of the new period to the old period can be used to update the gyro scale factor from its initial value. Thus, scale factor drift due to inexact serrodyne modulation amplitude control can be compensated in this manner.
In a article by H. C. Lefevre et al, it is suggested that it is possible to use phase steps with a step duration corresponding to the group propagation time through the fiber sensing coiil instead of an analog ramp.
Objects of the present invention include improve means for measuring the relative effective lengths traversed by counterpropagating beams of light.
According to the present invention, a current source and a capacitive integrated optic phase modulator are used in a circuit to drive said modulator, said circuit having means for measuring current provided to said modulator and said current being indicative of the relative effective lengths traversed by counterporpagating beams of light.
By using a current source and an integrated optic phase modulator as part of the drive circuit, the present invention takes advantage of the inherent capacitance of the phase modulator and generates a sawtooth voltage waveform having a relatively low amount of distortion without resorting to the use of very fast op amps. Measuring the current provided to the modulator results in an accurate measurement of the relative effective lengths traversed by counterpropagating beams of light.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing.